I am up to 11K grid stopper on the output tubes, and 100K on the last gain stage and concertina triodes.

I'm beginning to suspect the wide layout of the breadboard is creating large loops with the component leads.

I may be forced to build on a chassis to eliminate the oscillation.

I can hear it under some conditions when playing a guitar through the amp.
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I have a very cheap set of computer speakers (and very old) so I can't tell for sure about it being too bright but it sounds good to me.
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I think I will take a break, make a cabbage pie for dinner, and have a beer. After dinner I will trace out the schematic and update the Spice file and take a fresh look at it.

Thanks Anatoliy.

later..

Last edited by TheGimp; 9th July 2011 at 10:00 PM.
Reason: Clean up multiple posts.

I'm beginning to suspect the wide layout of the breadboard is creating large loops with the component leads.

This is possible. Mine works good with the low Gm tubes typically seen in audio amps. It can oscillate with high Gm things like mosfets. The copper screen under the modules helps somewhat.

I have been trying to follow this thread on my smart phone, but AT&T's coverage sucks in rural West Virginia. I have missed more than a few posts. Is your amp push pull? If so high Gm devices often do funny things as they go into or come out of cutoff. Try tinkering with the bias to see if the oscillation changes. I have seen a very similar oscillation in mosfet drivers.

For this exercize AMP 1.0 and AMP 2.0 were made on perf board. AMP 1.0 sounded like %#&@ until it blew up. AMP 2.0 was never completed since I had to leave town on 4 hours notice. I loaded Eagle on Sherri's netbook and attempted to lay out a PC board for a modified version of AMP 2.0. I should be able to finish it in the next few days. I don't know what will happen but if it works it will be the most complicated entry seen.

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Tubelab, it's 5 year mission. To explore strange new tubes, to seek out new circuits and topologies, to boldly go where no tube has gone before......

The output pentodes are 6DX8/6F4P. The pentode has a gm around 11,000. The Mu of the triode in the 6DX8/6F4P is 65 which I didn't think was exceptional compared to the 12AX7 at 100.

The first two stages are now the 6GH8A with the pentode stage first with a Gm of 7500. Mu of the triode is 46.

I can short the grid of the 6GH8A to ground with a 2" wire, and the output still oscillates. This indicates to me the problem is beyond the second gain stage, and must be in the 6DX8/6F4P, either in the third gain stage, the fourth stage Concertina or the output drivers themselves.

If I remove the coax to the gird of the third gain stage (6DX8/6F4P) and short the input to ground, there is still a 3V rms at 13.5KHz on the plate!

So the third gain stage oscillating. But why and how since the input is shorted to ground.

If the cathode were bypassed, it should effectively be shorted to ground and should not be an input, no?

I tried adding 56uF al and 1800pF Silver Mica in addition to the original 2.2uF WIMA. No change.

If I turn the output pot down to 50%, the oscillation stops.

If I add an equal resistance in series with the pot on the output, I can leave it turned up all the way and stop the oscillation. I have tried 100K and 1M pots and in both cases the oscillation stops around 50%. It seems the output from the pot is somehow feeding back (internal to the tube?) and causing the oscillation.

I just found this thread and took me a couple of days of reading through all the posts. I thought my thoughts may be useful as another reader's point of view of this competition. You will find my introduction in the appropriate section.

A little background here to explain my views: I have been involved with electronics on and off for 50 years. I have repaired and restored guitar amplifiers for a few decades both as a hobby and also as a business, professionally for paying customers.

Guitar Amp collectors are different from musicians and as long as the amp is restored to stock standard they will be happy. In fact that is what they want! Musicians are a different: The one and only thing that matters to them is SOUND, whether they are professional or amateurs! An amp and its sound will be perfect for on client while for another it will be rather unsatisfactory. Why? because we are individuals and have different preferences.

During my time, I have had a number of perfectly working amps brought to me for making changes to its sound. In my experience, making changes to some amps that made them sound different was not to difficult to do. For others it was not only difficult, but just about impossible at times. This may have been be due to the amp's design (schematic), its layout and/or its components characteristics.

So, I find it interesting that the main criteria of this competition is "who would build the amp" rather than what the amp sounds like. To me: would I build this amp to play blues, country, heavy metal, etc. is the question? If I would not than who cares how little it costs? It has no use for me!

An amp that will be judged as "ok" for all or many of the styles of music played will be not be satisfactory or really good for any of them! This is fact of life! It is no different from the car we buy. What is the main purpose of the car going to be? What I /we really want it to be perfect for? taking the rubbish to the tip? driving fast and sporty? off road driving? driving in most comfort and luxury? etc! I/You would buy a very different car for each of these purposes, would you not? I could have mentioned buying the tools we use. Either they are really specialised or they general good for nothing or little more than basics!

Another thought, once you have the winning design and people buy it/build it. How is it is going to be modified and/or repaired if needed? Should this not be a factor in judging the winner? I could mention a few very successful and highly thought after major amps (I am not going name them) that working on is an absolute pain in the .rs. If and when I was asked to work on them I had to point out to the clients it is going to cost more. Why? there are more labor cost to strip them down find and replace faulty parts and again to put them back together. For some you could not even test the changes without putting the whole thing back together. They were and still are great amps, but you just had to hope that they did not need servicing if you owned one!

Components quality (good, bad or indifferent) are also a key factor in the sound. Cabinets, speakers, OTs, tubes, caps, resistors, etc that are make up the amp will change its sound. If you do not supply all the factors that made your amp sound the way it does, hardly anyone will be able to recreate it just from the schematic!

In the same vain, you can build two guitars out of the same wood same components and while they will both sound great there will be subtle differences! Some people will hear them while other will not, but the differences will be there!

Another truth: The sound of both amps and guitars will change if you play them for a while!

I am looking forward to read more as this thread and the competition develops.